Grass mowing machines known as ZTR mowers have at least one independently powered drive wheel on each side of a frame. One drive wheel may be operated in a forward direction while the other drive wheel may be stopped or operated in reverse. Many ZTR mowers have a twin stick control system. Two control handles or sticks may be provided side-by-side, with each control handle or stick controlling one of the drive wheels. When both control handles or sticks are advanced together forwardly out of their neutral position, both drive wheels rotate forwardly to cause the mower to move forward. A ZTR mower may be steered by advancing one control handle or stick more than the other.
Typically, each control handle or stick on a ZTR mower may be linked to a pump arm of one of two separate hydraulic pumps, or of a dual hydraulic pump; i.e, a separate pump for each wheel. The control handle or stick may be used to move a pump swashplate through a direct linkage. For example, 10 degrees of operator input at the pump arm may yield 10 degrees of pump stroke.
ZTR mowers may be capable of operating at high ground speeds and mowing large amounts of grass while at high speeds. However, difficult mowing conditions may load the ZTR mower and cutting blades heavily enough to slow or even stall the engine if the operator does not reduce the ground speed. Such difficult mowing conditions may include very tall, wet, dense grass. Typically, the engine may strain and its speed may drop below a desired range. If the engine speed drops substantially, the rotational speed of the cutting blades also may drop below an optimal range, i.e., below a desired range of between about 2000 and about 2500 rpm. If the cutting blades rotate at speeds below their optimal range, the ZTR mower may provide poor or unacceptable cut quality.
In an attempt to maintain rotation of cutting blades at acceptable speeds, some ZTR mower operators may listen to the engine. If they notice the engine is straining, they may pull back on both control handles or sticks to slow the mower's forward speed and thereby reduce the engine load imposed to move the mower forward or in reverse. This can have the effect of allowing engine speed to increase back to its normal operating range which also speeds up the cutting blades. However, some operators do not slow the mower for adverse mowing conditions, but instead push both control handles or sticks all the way forward and leave them there during mowing, even if engine speed drops and cutting blades slow down. This can lead to poor cut quality under the conditions described above.
Some ZTR mowers may have mechanical or electrical controls that limit how far the control handles or sticks may be pushed forward during mowing. The controls may be set to a desired travel speed, or to a maximum travel speed. For example, U.S. Pat. No. 6,161,637 relates to a twin stick control system that the operator may preset to a travel speed, and also may be speeded up and automatically returned later to same preset speed. However, this speed control system does not reduce the travel speed in adverse mowing conditions so that the rotational speed of the cutting blades stays within the optimal range.
An apparatus is needed to automatically reduce the travel speed of a ZTR mower in heavy grass or other adverse mowing conditions. A ZTR mower is needed that will rotate the cutting blades within an optimal range even when the engine is straining.